Many electronic systems use internal clocks which are required to be phase-aligned with an external reference clock, such as the clock embedded in an asynchronous incoming data stream. Phase alignment is important for data to be exchanged reliably between circuits. Clock signals are often provided by Phase-Locked Loop (PLL) circuitry on programmable logic devices, and the PLL circuits produce output signals that are continually adjusted to maintain a constant frequency and a constant phase relationship with an input reference signal.
A multiple-phase clock system provides multiple signals, each signal with a different phase, that can be used to alternately enable and disable circuits. In a multiple-phase clock system, functions involving more than one operation may be completed during a given clock cycle. However, if the clock signals have phase errors, the operations performed using the off-phase clock signals may fail due to a timing error. Unfortunately, some architectures assume that the input clocks are exactly 90° phase shifted from each other.
Possible causes for clock phase spacing error include device random mismatch, routing mismatch, and imbalanced clock loading. Most high-speed clock buffers use short channel length devices for speed. Since device random variation is inversely proportional to device gate area, the device matching between clock buffers is negatively affected. Further, for multi-gigahertz clock routings, both resistance and capacitance are important in order to match clock propagation delays, and any imperfection in the process will create clock phase spacing errors. In addition, top level metal routing can cause mismatches on clock routings. If there are unused clock phases, dummy loads need to be added to match the loading. Unfortunately, dummy loads cannot mimic the active load completely since dummy clock paths are usually disabled to save power.
Polyphase filters, also called I/Q generators, have been used to address clock phase spacing error issues. Polyphase filters, however, suffer from their narrow correction bandwidths. In addition, polyphase filters are not practical for low frequency applications due to the large floating capacitors needed in the filters.
It is in this context that embodiments arise.